Resource‐allocation tradeoffs in caddisflies facing multiple stressors

The replacement of native forests by exotic tree monocultures, such as those of Eucalyptus, decreases the quality of leaf litter inputs to streams and often reduces riparian cover, which can elevate water temperature. The combined effects of these stressors on the survival and performance of detritivores may be important, as detritivore species loss leads to reduced litter breakdown, a key ecosystem process. Potential loss of cased caddisfly larvae is of particular concern because they are the predominant detritivores in many streams, they are sensitive to warming, and they expend energy on building and carrying their cases, which may be an added burden under times of stress. In a microcosm experiment, we tested whether (i) poor‐quality Eucalyptus globulus litter impaired case construction by larvae of Sericostoma pyrenaicum (due to preferential allocation of the scarcer available energy to larval fitness) compared to high‐quality Alnus glutinosa litter; (ii) whether this effect was enhanced by higher temperatures (15 vs. 10°C) resulting in faster metabolism and greater energy expenditure; but (iii) reduced in the presence of chemical cues from a predatory fish (due to greater investment in more protective cases). We found that Eucalyptus had lethal and sublethal effects on larval caddisflies, increasing mortality, reducing growth, and impairing case construction, compared to larvae fed Alnus. Temperature did not reinforce the effects of exotic litter on case construction, but predator chemical cues triggered the construction of more protective cases (i.e., longer and better cemented) despite the lower resource quality, providing evidence for environmentally mediated resource‐allocation tradeoffs.     

Seasonal availability of resources and habitat degradation for the western tree-hole mosquito, <Emphasis Type="Italic">Aedes sierrensis</Emphasis>

The nutrient base of aquatic tree-hole communities is derived from leaf litter, benthic detritus, and water flowing down the tree trunk (stemflow water). Previous studies in eastern North America with the mosquito, Aedes triseriatus, have identified leaf litter as a major and stemflow water as a minor source of mosquito nutrition, but did not consider the role of the benthic detritus or how the aggregate or relative contribution of these sources of mosquito nutrition changed during the year. We use the leaf litter, benthic detritus, and stemflow water from tree holes in western Oregon (USA) to determine how these substrates affect mass at metamorphosis, biomass yield, and fitness (cohort replacement rate; R ₀) of the mosquito, Aedes sierrensis, through both natural and simulated winters, the normal growing season for larvae in tree holes. We found that fresh leaf litter constitutes the major determinant of mosquito fitness by a factor of >15:1 over any other substrate taken directly from tree holes in nature. The other substrates, including the benthic detritus, individually make only a meager contribution to mosquito fitness but, when added to the leaf litter, can sustain yield and improve fitness at high, limiting larval densities. Nutritional quality of tree-hole substrates declines by >90% from early (fall) to late (spring) in the larval growing season. At both times of year, the coarse or fine detritus provide minor resources, and stemflow water provides no detectable contribution to mosquito nutrition. The resources in the litter are not transported during the year to the benthic detritus; rather, these resources are either exploited by mosquitoes when they first become available, or they deteriorate and become progressively more unavailable to them. Growth and development of A. sierrensis feeding on dried and reconstituted tree-hole contents during a 6-month simulated winter in the laboratory showed: (1) the same relative contributions of leaf litter, benthic detritus, and stemflow water to mosquito nutrition, (2) that the winter deterioration of substrate quality is a direct consequence of microbial decomposition, and (3) that pre-emptive competition from pre-existing A. sierrensis greatly increases substrate deterioration. We conclude that the progressive winter deterioration of larval resources in combination with the dry summers of western North America are the most likely environmental factors that limit species diversity in tree holes and that have selected for early recruitment (autumnal hatching) of A. sierrensis and for its univoltine life cycle from Mexico to Canada. Received: 28 December 1999 / Accepted: 10 April 2000     

Sericostoma vittatum (Trichoptera) Larvae Are Able to Use Pine Litter as an Energy Source

We tested the effect of conditioning time of Pinus sylvestris litter on food consumption and growth of laboratory‐reared Sericostoma vittatum larvae. In all cases, larval mass was positively correlated with consumption rates but negatively with growth rates and gross production efficiencies. Conditioning time (4, 8 or 30 weeks) had an effect on feeding rates: they were lowest for the least conditioned pine litter, but no effect of conditioning time on growth rates was observed. Contrary to previous reports on conifer litter use by aquatic detritivores, our results indicate that S.vittatum larvae were able to transform the highly refractory pine litter into secondary production even when it was conditioned for only 4 weeks. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nitrate sequestration by corticolous macrolichens during winter precipitation events

Nitrogen is an essential nutrient in the biogeochemistry of forested ecosystems. The influence of canopy lichens on the winter biogeochemistry of nitrate in broadleaved deciduous forests is examined and it is hypothesized that nitrate sequestration will not differ between winter precipitation events. Rejection of this hypothesis would mean that meteorological conditions of winter precipitation events have a detectable influence on nitrate sequestration by canopy lichens and nitrate input to the forest floor. Canopy lichens of the genus Parmelia were found to influence winter nitrate stemflow inputs to forest soils differentially. Epiphytic lichens on an individual Carya glabra Mill. (pignut hickory) canopy tree, centrally located within the stand of an open deciduous forest, actively sequestered nitrate leached from the tree's woody frame, lowering aqueous stemflow inputs at the tree base. The quantities of nitrate sequestered by corticolous lichens during the 2 February 1999 mixed-precipitation event were significantly greater than those during all other precipitation events examined. Greater rates of nitrate uptake during the 2 February 1999 event may be attributed to (1) its intermediate rain intensity, which would have soaked the lichen thalli in a nutrient-rich bath, and (2) an air temperature range between -2 degrees C and 8 degrees C that would have increased viscosity and surface tension of stemflow drainage, thereby decreasing stemflow velocity and increasing the contact time of stemflow water on the lichen thalli. Other precipitation events were either too cold to promote metabolic activity by canopy lichens or too warm and intense for an optimal contact time of stemflow with lichen thalli, resulting in lower quantities of nitrate sequestered. Meteorological conditions of winter precipitation events have been documented to influence sequestration of nitrate by corticolous lichens and decrease aqueous stemflow inputs to the forest floor of broadleaved deciduous forests.     

Anthropogenic changes to leaf litter input affect the fitness of a larval amphibian

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Maternal diet and larval diet influence survival skills of larval red drum Sciaenops ocellatus

Larval red drum Sciaenops ocellatus survival, turning rate, routine swimming speed, escape response latency and escape response distance were significantly correlated with essential fatty‐acid (EFA) concentrations in eggs. Of the five traits that varied with egg EFA content, two (escape response latency and routine swimming speed) were significantly different when larvae were fed enriched diets compared with the low fatty‐acid diet, indicating that the larval diet can compensate for some imbalances in egg composition. Turning rate during routine swimming and escape response distance, however, did not change when larvae predicted to have low performance (based on egg composition) were fed an enriched diet, indicating that these effects of egg composition may be irreversible. Escape response distances and survival rates of larvae predicted to perform well (based on egg composition) and fed highly enriched diets were lower than expected, suggesting that high levels of EFA intake can be detrimental. Altogether, these results suggest that both maternal diet, which is responsible for egg EFA composition, and larval diet may play a role in larval survivorship and recruitment.     

EFFECTS OF ELEVATED ATMOSPHERIC CO(2) ON WATER CHEMISTRY AND MOSQUITO (DIPTERA: CULICIDAE) GROWTH UNDER COMPETITIVE CONDITIONS IN CONTAINER HABITATS

We investigated the direct and indirect effects of elevated atmospheric CO(2) on freshwater container habitats and their larval mosquito occupants. We predicted that a doubling of atmospheric CO(2) would (1) alter the chemical properties of water in this system, (2) slow degradation of leaf litter, and (3) decrease larval growth of Aedes albopictus (Skuse) mosquitoes raised on that litter under competitive conditions. Effects of elevated CO(2) on water quality parameters were not detected, but the presence of leaf litter significantly reduced pH and dissolved oxygen relative to water-filled containers without litter. Degradation rates of oak leaf litter from plants grown under elevated CO(2) atmospheres did not differ from breakdown rates of litter from ambient CO(2) conditions. Litter from plants grown in an elevated CO(2) atmospheres did not influence mosquito population growth, but mosquito production decreased significantly with increasing larval density. Differences among mosquito density treatments influenced survivorship most strongly among male Ae. albopictus and time to emergence most strongly among females, suggesting fundamental sex-determined differences in response to competition. Results of this and other studies indicate that direct and indirect effects of doubled atmospheric CO(2) are minimal in artificial containers with freshwater.     

华北暖温带山地落叶阔叶混交林的茎流研究

测定华北暖温带山地落叶阔叶混交林的树干茎流量及其元素含量,并比较了不同树种是的差异。通过８２次孤立降雨的测量,得到了辽东栋、棘皮桦、五角枫、大叶白腊和蒙椴５种主要树种及整个林分茎流量Ｓ与降雨Ｐ的关系,经多种方程拟合,仍以直线方程为最佳。降雨水质分析的结果给出了Ｎ、Ｐ、Ｋ、Ｃａ、Ｍｇ、Ａｌ、Ｓ元素各次测量的美元浓度及其变异系数,以及年输入量的估测值,茎流元素含量的分析结果表明,５种乔木中,以棘皮桦的     

Effects of nutrition and density in Culex pipiens

Mosquito larvae face numerous biotic and abiotic challenges that affect their development and survivorship, as well as adult fitness. We conducted two experiments under semi‐natural conditions to evaluate the effects of intraspecific competition, nutrient limitation and sub‐lethal doses of malathion on individual life history traits in adult Culex pipiens (Diptera: Culicidae). In the first experiment, larvae of Cx. pipiens were reared at different intraspecific densities and exposed to sub‐lethal doses of malathion. In the second experiment, different intraspecific densities of Cx. pipiens larvae were reared under conditions of low or high larval nutrients, and subsequent adults were fed on either water or 10% sucrose solution. Malathion treatment had relatively minor effects compared with density, which had significant negative effects on development rate, survivorship to adulthood, body size (wing length) and longevity. As larval density increased, a sex ratio distortion in survivorship to adulthood emerged, in which a bias towards males was apparent. Nutrient‐rich larval environments alleviated, in part, the effects of increasing density and extended the lifespan of mosquitoes fed on water and 10% sucrose. Density‐dependent alterations in adult longevity attributable to the larval environment are complex and show contrasting results depending on interactions with other environmental factors. This study suggests that larval resource availability and competition influence Cx. pipiens population growth correlates and have lasting effects on traits that relate to a mosquito's ability to vector pathogens.     

Herbivore and detritivore effects on rainforest plant production are altered by disturbance

Consumer effects on rainforest primary production are often considered negligible because herbivores and macrodetritivores usually consume a small fraction of annual plant and litter production, even though consumers are known to have effects on plant production and composition in nontropical systems. Disturbances, such as treefall gaps, however, often increase resources to understory food webs, thereby increasing herbivory and feeding rates of detritivores. This increase in consumption could lead to more prominent ecosystem‐level effects of consumers after disturbances, such as storms that cause light gaps. We determined how the effects of invertebrate herbivores (walking sticks) and detritivores (litter snails) on understory plant growth may be altered by disturbances in a Puerto Rican rainforest using an enclosure experiment. Consumers had significant effects on plant growth, but only in light gaps. Specifically, herbivores increased plant growth by 60%, and there was a trend for detritivores to reduce plant growth. Additionally, plant biomass tended to be 50% higher with both consumers in combination, suggesting that herbivores may mediate the effects of detritivores by altering the resources available to detritivore food webs. This study demonstrates that disturbance alters the effects of rainforest consumers, and, furthermore, that consumer activity has the potential to change rainforest successional processes.     

Antagonistic, stage-specific selection on defensive chemical sequestration in a toxic butterfly

Larvae of the pipevine swallowtail ( Battus philenor ) sequester toxic alkaloids called aristolochic acids from their Aristolochia host plants, rendering both larvae and adults chemically defended against most predators. Using a chemically controlled artificial diet, we observed substantial among-family variation in sequestration ability and larval developmental rate in a population occurring in central Texas. Early instar larvae from families that sequester greater amounts of aristolochic acid showed increased survivorship in a field experiment in which cohorts from each family were exposed to natural predators, whereas among-family variation in growth rate did not predict survivorship. Conversely, the aristolochic acid content of adult butterflies was negatively correlated with adult fat content, a fitness correlate. Sequestration ability positively affects the probability of larval survivorship, but at the cost of adult fat content. The costs and benefits of aristolochic acid sequestration vary during the course of the butterfly's development, and these antagonistic selection pressures may explain why variation in sequestration ability persists in wild populations.     

Nutritional quality of leaf detritus altered by elevated atmospheric CO2: effects on development of mosquito larvae

1. Populus tremuloides leaf litter was produced under elevated (ELEV = 720 ppm) and ambient (AMB = 360 ppm) atmospheric CO₂ conditions. Leaf chemical quality was significantly altered by CO₂ enrichment. ELEV leaves had significantly higher concentrations of phenolic compounds and lignins, and higher C : N ratios than AMB. 2. Leaf litter was incubated in a headwater stream for 14 days to become colonised by microorganisms; aquatic bacterial productivity was significantly lower on ELEV than on AMB leaf litter. Colonised leaves were fed to four species of detritivorous mosquito larvae to assess their survivorship and development rates. 3. Larval mortality was 2.2 times higher for Aedes albopictus fed ELEV litter when compared with AMB. Although mortality of A. triseriatus, A. aegypti and Armigeres subalbatus was not affected by treatment, larval development rate was delayed by 78, 25 and 27%, respectively, when fed ELEV litter. 4. Increased mosquito mortality and/or delayed larval development rates are more likely to have negative implications for food web structure and productivity in ecosystems where immature stages of mosquitoes are an important food source of predators.     

Effect of carnivory on larvae and adults of a detritivorous caddisfly, Lepidostoma complicatum: a laboratory experiment

Lepidostomatid caddisfly larvae are typical detritivores, but they occasionally eat small dead animals. A laboratory feeding experiment was conducted with Lepidostoma complicatum (Kobayashi) larvae using two different feeding treatments: leaves or leaves and dead chironomids. L. complicatum larvae showed significantly higher growth rates and adult emergence weight and a significantly earlier emergence for the leaves and dead chironomids treatment than for the leaves alone treatment. However, the adult emergence rate was not different between the two feeding treatments. Thus, it is apparent that ingestion of dead chironomids by detritivorous L. complicatum larvae positively influences larval growth rates, adult emergence weight, and larval development time.     

Tree litter functional diversity and nitrogen concentration enhance litter decomposition via changes in earthworm communities

Biodiversity is a major driver of numerous ecosystem functions. However, consequences of changes in forest biodiversity remain difficult to predict because of limited knowledge about how tree diversity influences ecosystem functions. Litter decomposition is a key process affecting nutrient cycling, productivity, and carbon storage and can be influenced by plant biodiversity. Leaf litter species composition, environmental conditions, and the detritivore community are main components of the decomposition process, but their complex interactions are poorly understood. In this study, we tested the effect of tree functional diversity (FD) on litter decomposition in a field experiment manipulating tree diversity and partitioned the effects of litter physiochemical diversity and the detritivore community. We used litterbags with different mesh sizes to separate the effects of microorganisms and microfauna, mesofauna, and macrofauna and monitored soil fauna using pitfall traps and earthworm extractions. We hypothesized that higher tree litter FD accelerates litter decomposition due to the availability of complementary food components and higher activity of detritivores. Although we did not find direct effects of tree FD on litter decomposition, we identified key litter traits and macrodetritivores that explained part of the process. Litter mass loss was found to decrease with an increase in leaf litter carbon:nitrogen ratio. Moreover, litter mass loss increased with an increasing density of epigeic earthworms, with most pronounced effects in litterbags with a smaller mesh size, indicating indirect effects. Higher litter FD and litter nutrient content were found to increase the density of surface‐dwelling macrofauna and epigeic earthworm biomass. Based on structural equation modeling, we conclude that tree FD has a weak positive effect on soil surface litter decomposition by increasing the density of epigeic earthworms and that litter nitrogen‐related traits play a central role in tree composition effects on soil fauna and decomposition.     

Stemflow nutrient inputs to soil in a successional hardwood forest

Stemflow and throughfall from a regenerating (8-year-old) southern Appalachian hardwood forest were collected to examine the relative importance of tree bole nutrient leaching in response to acid deposition. Samples from nine (2 m²) stemflow collection plots were analyzed for four dormant season and 11 growing season rainstorm events. Results showed that, relative to throughfall fluxes, stemflow accounted, on average, for approximately 8.5% of total water reaching the forest floor during both dormant and growing season storms. Relative to foliar leaching, K-, SO₄-, and PO₄ ions appear to be the most easily leached ions from young tree stems. Proportional nitrate and base cation stemflow fluxes increased significantly (p<0.05) with growing-season storm-event duration, suggesting that the stemsurface nutrient pool is depleted by precipitation more slowly than the foliar pool. On average, proportional stemflow fluxes of SO₄ (12%) and K (14%) were consistently higher than reported maximum values for more mature forest stands, which indicates that small-scale stemflow inputs of ions such as these to the forest floor may be important in early successional ecosystems.     

GEOGRAPHIC VARIATION IN LIFE-HISTORY TRAITS OF THE ANT LION,MYRMELEON IMMACULATUS: EVOLUTIONARY IMPLICATIONS OF BERGMANN'S RULE

In eastern North America, body size of the larval ant lion Myrmeleon immaculatus increases from south to north, following Bergmann's rule. We used a common-garden experiment and a reciprocal-transplant experiment to evaluate the effects of food and temperature on ant lion growth, body size, and survivorship. In the laboratory common-garden experiment, first-instar larvae from two southern (Georgia, South Carolina) and two northern (Connecticut, Rhode Island) populations were reared in incubators under high- and low-food and high- and low-temperature regimes. For all populations, high food increased final body mass and growth rate and decreased development time. Growth rates were higher at low temperatures, but temperature did not affect larval or adult body mass. Survivorship was highest in high-food and low-temperature treatments. Across all food and temperature treatments, northern populations exhibited a larger final body mass, shorter development time, faster growth rate, and greater survivorship than did southern populations. Results were similar for a field reciprocal-transplant experiment of third-instar larvae between populations in Connecticut and Oklahoma: Connecticut larvae grew faster than Oklahoma larvae, regardless of transplant site. Conversely, larvae transplanted to Oklahoma grew faster than larvae transplanted to Connecticut, regardless of population source. These results suggest that variation in food availability, not temperature, may account for differences in growth and body size of northern and southern ant lions. Although northern larvae grew faster and reached a larger body size in both experiments, northern environments should suppress growth because of reduced food availability and a limited growing season. This study provides the first example of countergradient selection causing Bergmann's rule in an ectotherm.     

Effects of transgenic American chestnut leaf litter on growth and survival of wood frog larvae

Biotechnology offers a new approach for the restoration of tree species affected by exotic pathogens; however, nontarget impacts of this novel strategy on other organisms have not been comprehensively assessed. We evaluated the effect of transgenic American chestnut (Castanea dentata) leaf litter on the growth and survival of larval wood frogs (Lithobates sylvaticus), a forest‐dwelling amphibian species widely sympatric with American chestnut, that forage almost entirely on periphyton and litter detritus that accumulate in temporary vernal pools in forests. We reared wood frog larvae on Castanea leaf litter (American chestnut genetically engineered for blight tolerance, nontransgenic American chestnut, Chinese chestnut [Castanea mollissima], and an American–Chinese chestnut hybrid) and litter from two non‐Castanea, nontransgenic “control” tree species, coupled with two levels of supplementary food. We observed no differences in growth or survival of wood frog larvae reared on transgenic versus nontransgenic American chestnut leaves. Without supplementary food, wood frog larvae provided leaves from American chestnut (both types) developed faster and grew larger than those exposed to other leaf litter treatments. Results of this study provide preliminary evidence that (1) American chestnut may have formerly been an important source of food for forest‐dwelling amphibians and (2) transgenic American chestnut litter generated as part of chestnut restoration efforts is unlikely to present direct novel risks to developing amphibian larvae in the forest environment.     

Symbiont transmission and reproductive mode influence responses of three Hawaiian coral larvae to elevated temperature and nutrients

Elevated temperatures and nutrients are degrading coral reef ecosystems, but the understanding of how early life stages of reef corals respond to these stressors remains limited. Here, we test the impact of temperature (mean ~ 27 °C vs. ~ 29 °C) and nitrate and phosphate enrichment (ambient, + 5 µM nitrate, + 1 µM phosphate and combined + 5 µM nitrate with 1 µM phosphate) on coral larvae using three Hawaiian coral species with different modes of symbiont transmission and reproduction: Lobactis scutaria (horizontal, gonochoric broadcast spawner), Pocillopora acuta (vertical, hermaphroditic brooder) and Montipora capitata (vertical, hermaphroditic broadcast spawner). Temperature and nutrient effects were species specific and appear antagonistic for L. scutaria and M. capitata, but not for P. acuta. Larvae survivorship in all species was lowest under nitrate enrichment at 27 °C. M. capitata and L. scutaria survivorship increased at 29 °C. However, positive effects of warming on survivorship were lost under high nitrate, but phosphate attenuated nitrate effects when N/P ratios were balanced. P. acuta larvae exhibited high survivorship (> 91%) in all treatments and showed little change in larval size, but lower respiration rates at 29 °C. Elevated nutrients (+N+P) led to the greatest loss in larvae size for aposymbiotic L. scutaria, while positive growth in symbiotic M. capitata larvae was reduced under warming and highest in +N+P treatments. Overall, we report a greater sensitivity of broadcast spawners to warming and nutrient changes compared to a brooding coral species. These results suggest variability in biological responses to warming and nutrient enrichment is influenced by life-history traits, including the presence of symbionts (vertical transmission), in addition to nutrient type and nutrient stoichiometry.

     

Utilization of larval and pupal detritus by Aedes aegypti and Aedes albopictus

The utilization of detritus sources by mosquito larvae during development may significantly affect adult life history traits and mosquito population growth. Many studies have shown invertebrate carcasses to be an important detritus source in larval habitats, but little is known regarding how invertebrate carcasses are utilized by mosquito larvae. We conducted two studies to investigate the rate of detritus consumption and its effect on larval development and life history traits. Overall, we found that Aedes aegypti and Aedes albopictus larvae rapidly consumed larval detritus, while pupal detritus was consumed at a significantly slower rate. We also found that the consumption of larval detritus significantly increased larval survivorship and decreased male development time but did not significantly influence female development time or pupal cephalothorax length for either sex. Our results suggest that the direct consumption of larval detritus can support the production of adults in larval habitats that lack allochthonous detritus inputs or where such organic inputs are insufficient. These studies indicate that different forms of invertebrate detritus are utilized in distinct ways by mosquito larvae, and therefore different forms of invertebrate detritus may have distinct effects on larval development and adult life history traits.     

Drought and the organization of tree-hole mosquito communities

Summary In southeastern North America (North Florida, USA), the duration, frequency, and timing of drought differentially affect the survivorship of pre-adult tree-hole mosquitoes. Drought affects survivorship both by the direct action of dehydration on developing larvae and pupae and by the indirect modulation of predation. The drought-susceptible species, Toxorhynchites rutilus, Orthopodomyia signifera, and Anopheles barberi co-occur in more permanent holes that are larger, with larger, more vertical openings, lower down in larger trees, and contain darker water with higher conductivity, pH, and tannin-lignin content than the holes occupied by Aedes triseriatus that has drought-resistant eggs and rapid larval development. Ovipositing mosquitoes cue on physical and chemical attributes of tree holes independently of host tree species. These same attributes differ among drought-prone and drought-resistant holes but mosquitoes track these attributes more faithfully than the attributes predict tree-hole stability.